1. Field of the Invention
The present invention relates to a control device for a bicycle derailleur comprising a fixing member, control means for effecting a change speed, a reciprocating member reciprocatively supported by the fixing member and operatively connected to the derailleur to be movable by operating the control means in forward direction for shifting a chain from a smaller sprocket to a larger sprocket and in a backward direction for shifting the chain from the larger sprocket to the smaller sprocket while being urged in the backward direction, and a retaining mechanism acting as a one-way mechanism for permitting the reciprocating member to move in the forward direction and preventing the same from moving in the backward direction thereby to position and maintain the reciprocating member.
2. Description of the Prior Art
One example of the known retaining mechanisms for derailleur control devices of this type is shown in GB 2,169,065 A disclosing the reciprocating member pivotally supported by the fixing member, recesses defined in a lower face of the reciprocating member for stepwise positionally maintaining the reciprocating member, and further recesses defined in an upper face of the reciprocating member for stepwise releasing the reciprocating member. A ratchet mechanism is structured to include a retaining pawl directing upwardly and defined in a button constantly urged upwardly for engaging the lower recesses, and a restraining pawl directing downwardly and defined in the button for engaging the upper recesses and effecting a stepwise release when pushing down the button. The reciprocating member is maintained in one set position for each of a plurality of speeds in the forward and backward directions by alternately engaging two pairs of pawls and recesses.
When shifting the chain from the smaller sprocket to the larger sprocket, it is necessary to overshift a chain portion supported by the derailleur to the larger sprocket beyond a position corresponding to a sprocket to be engaged (FIG. 12(a)).
When shifting the chain from the larger sprocket to the smaller sprocket, it is necessary to overshift the chain portion supported by the derailleur to the smaller sprocket beyond the position corresponding to the sprocket to be engaged (FIG. 12(b)).
It should be noted that the overshift amount is substantially constant when the chain is shifted from the larger sprocket to the smaller sprocket while the overshift amount varies when the chain is shifted from the smaller sprocket to the larger sprocket. This is because the tension of the chain is changed significantly when shifting from the smaller sprocket to the larger sprocket. The more increased the tension is, the more increased the overshift amount is.
However, according to the conventional structure, the reciprocating member is maintained in only one set position for each speed and not in an overshift position when the chain is shifted from the smaller sprocket to the larger sprocket, which does not allow a smooth change speed. In addition, when the reciprocating member is returned to the backward direction, the pawl is engaged with one of the recesses for the stepwise release thereby to prevent a plurality of speed changes and a quick speed change control.